Where can I change mixing of an action in NLA?
The relevant setting is Blend In/Out. To ignore keyframes in strips below the active one, turn off Auto Blend In/Out, and set both blend values to 0:



More generally I suggest, for bones that will be animated in a layer, to avoid keyframing it in strip actions in all other layers. Or do it in such a way that the strips don't overlap. That'd save you some headache when the animation mixing gets more complex, especially if there are semi-overlapping action strips. Not used carefully, action blending could cause jerky movement that's hard to tweak.